Taper pins are an old machinists' trick for accurately and repeatably aligning parts.
In a woodworking book I recently picked up (more about that in the project article) they adopted taper pins as a solution for creating more accurate woodworking jigs.
Since I'm a little short on sources for the "special tapered reamer" they require, I decided to design this thing to allow me to use the tapered reamer I had on hand ($3 from Harbor Freight)
If that's the reamer you have, you can download the .stl directly. If not, you can go ahead and design your own pin.
Much more info about the book in question, taper pins, and why you need them are in the original website article, here: